A fuel cell is a new electricity generating device that uses a fuel and an oxidant to create electricity by an electrochemical Process. The fuel cell is continuously supplied with a fuel and an oxidant from the external BOP System on the other hand, A general electric cell is exhausted from electrochemical reaction within the electric cell.
A fuel cell includes a molten carbonate fuel cell (MCFC) operating at 500˜700° C., a phosphoric acid electrolyte fuel cell operating at 200° C., an alkaline electrolyte fuel cell operating at 100° C. or less, and a polymer electrolyte fuel cell.
The polymer electrolyte fuel cell is divided into a proton exchange membrane fuel cell (PEMFC) using a hydrogen gas as a fuel, and a direct methanol fuel cell (DMFC) using liquid methanol as a fuel.
Like other types of fuel cells, the MCFC shows high efficiency, an environmental friendliness, and a desirable modulation characteristic, and requires a reduced occupied space. Furthermore, the MCFC is operated at a high temperature of 650° C., and thus has advantages which are not expected from a low-temperature type fuel cell (such as a phosphoric acid fuel cell or a polymer fuel cell), as described below.
In other words, the MCFC is advantageous in economical efficiency because it can employ an inexpensive nickel electrode instead of a platinum electrode due to a high-speed electrochemical reaction at a high temperature. Furthermore, the nickel electrode can use, as a fuel, even carbon monoxide which functions as a poisonous material in the platinum electrode, through a Hydrogen gas transfer reaction. Thus, in the MCFC, various fuels such as coal gas, natural gas, methanol, biomass, may be selectively employed.
Also, high-temperature waste heat can be recovered and used through a bottoming cycle using an HRSG (Heat Recovery Steam Generator), thereby improving the heat efficiency of the entire electricity generating system by about 60% or more.
Meanwhile, lithium carbonate (Li2CO3) and potassium carbonate (K2CO3), used as raw materials for an electrolyte in a MCFC, have very high melting points, respectively. However, when they are mixed in a predetermined ratio, and made into powder with an average particle size (D50) of 50 μm or less, the melting point is lowered to about 500° C. according to the composition. Accordingly, a carbonate electrolyte used for the MCFC is in a solid state at a room temperature, but is molten and impregnated into matrix pores at an operating temperature of 650° C., thereby causing an electrochemical reaction between an electrode in a solid state with a reactive gas in a gas state.
In a conventional MCFC, a dry-electrolyte powder is molten at a high operating temperature, thereby reducing the height of the stack. This causes damage to configuration elements by an impact, and disadvantages in manufacturing peripheral devices for insulation and piping. Furthermore, the melting of the dry-electrolyte powder increases a contact resistance, and also the contraction of the electrode causes damage due to a thermal shock of a matrix.
Meanwhile, in order to solve the above described problems, Korean Patent Application No. 1999-0034894, titled “A method for impregnating an electrolyte for molten carbonate fuel cell” discloses a method for electrolyte-impregnated cathode by using an electrolyte green sheet manufactured by tape casting. Also, Korean Patent Application No. 2007-0135794 titled “Fabrication method of large-sized electrolyte-containing electrodes for MCFC” discloses a method for impregnating an electrolyte in a cathode by using dry-electrolyte eutectic carbonate powder.
However, these methods require a complicated manufacturing process such as a long-time removal of an organic compound in an oxidizing atmosphere at 450° C., and temperature rise in a reducing atmosphere. Furthermore, a residual carbon is formed on a sintered cathode surface, or causes a defect on the surface. Thus, the methods have a problem in that uniformity and manufacturing efficiency are significantly reduced in a large-sized electrode in mass production.
Also, in manufacture of a large-area electrode according to the methods, electrode contraction occurring during electrolyte impregnation causes the bending and cracking of a large-area cathode. Thus, it is required to solve these problems.